Optical recording elements having recording layers containing mixtures of no k metallized formazan and cyanine dyes

ABSTRACT

An optical recording element is disclosed. The element has a transparent substrate and on the surface of said substrate, a recording layer and a light reflecting layer wherein recording layer comprises a mixture of a metallized formazan dye and cyanine dye and the unrecorded layer is such that the real part of the refractive index(n) at 780 nm is greater than 1.8 and the imaginary part (k) is less than 0.15. The metallized formazan dye has a k value of 0.00 to 0.02.

FIELD OF THE INVENTION

The present invention relates to optical recording elements, includingoptical recordable compact disks.

BACKGROUND OF THE INVENTION

There are many known types of optical recording elements. One of thecurrently popular elements is the compact disk (CD). Digital informationis stored in the form of high optical density marks or pits on anotherwise reflective background. In this format, the optical informationis most often in the form of read only memory or ROM. Opticalinformation is not usually recorded in real time but rather is producedby press molding. In a typical process, the optical recording substrateis first press molded with a master containing the digital informationto be reproduced. The thus formed information is then overcoated with areflective layer and then with an optional protective layer. In thoseareas having the deformations or pits, the optical density is higherthan in those areas not having the deformations.

It is desirable to produce recordable optical recording elements which,when recorded, produce a record that mimics the conventional CD on readout. Recording and read out is at about 780 nm. In this manner,information can be added to the CD and the CD can be used on aconventional CD player.

One element of this type is the so called "Photo CD". In the system inwhich this element is used, conventional photographic film is firstprocessed in a conventional manner. Then, the images from the film aredigitized and the digital information is recorded in a CD readable formon the optical recording element (Photo CD). Images can then be playedback on a CD-type player into a conventional television. Since theelement has a capacity for a number of digitized images that is greaterthan the number of images on a typical roll of consumer film, it isanticipated that the user will want to add images to a preexisting CD.

These recordable optical recording elements consist of a polycarbonatesubstrate containing a continuous tracking groove. An organic dye layer(recording layer) is solvent coated onto the grooved substrate.Fabrication is completed by coating a reflector layer, a protectivelayer, and a label in a fashion similar to a conventional CD digitalaudio disc or CD-ROM disc. In the recordable element the digitalinformation is written into the dye layer with the focused beam of adiode laser operating in the near infrared region of the spectrum.

Commercially useful recordable optical recording elements have stringentrequirements. The recording layer must have the required reflectivityand must also be able to couple with incident laser irradiation toprovide features having adequate optical contrast. The layer must alsohave good stability towards light, heat and humidity for acceptableshelf life (incubation stability). Since the Photo CD is a consumerproduct, it must be capable of withstanding extreme environments.Between the time the original images are recorded on the Photo CDelement and the time subsequent images are recorded, the element mightbe placed in strong sunlight.

In addition, the optical properties of the recording layer (its'refractive index) must be finely tuned especially as regards `k` (theimaginary component of the index of refraction) which controls theabsorption of light at the writing laser wavelength. K must be finitebut low to insure a balance between coupling and reflectivity.

Experimentally, it is difficult to find a single dye with theappropriate value of k. It is common in the art to combine symmetricalcyanine dyes (usually at least two dyes, one with a high k and one witha low k) to meet the k requirements. See U.S. Pat. No. 5,391,413.Indodicarbocyanine dyes have been used frequently. However, these dyesoften have less than the desired light stability and will in fact fadeto an unusable state after only a few days of intense sunlight exposure.

Optical recording layers containing metallized formazan dyes aredisclosed in U.S. Pat. No. 5,294,471. These recording layers do not havesufficient capability to couple with incident laser irradiation to formthe necessary contrast and sensitivity.

U.S. Pat. No. 5,547,728 discloses optical recording elements havingrecording layers comprising a mixture of metallized formazan dye and acyanine dye. Such elements are useful but need improvements in recordingsensitivity, incubation stability and light stability. Most of theexpressly disclosed metallized formazans therein have a strong electronwithdrawing group (typically NO₂). This results in a relatively largeabsorption at 780 nm. As a result, those formazans have, as part oftheir complex refractive index, high k values (about 0.15). Thisrelatively large k value has a negative impact on reflectivity when usedwith other recording layer dyes also having high k values.

Thus, there is a continuing need for optical recording materials thathave the necessary optical characteristics such that they are CDcompatible, can couple with incident laser irradiation to form featureswith sufficient contrast and yet are light and dark (incubation) stable.

SUMMARY OF THE INVENTION

In accordance with the present invention there is provided an opticalrecording element having a transparent substrate and on the surface ofsaid substrate, a recording layer and a light reflecting layer whereinthe recording layer comprises a mixture of a metallized formazan dye andcyanine dye and the unrecorded layer is such that the real part of therefractive index (n) at 780 rm is greater than 1.8 and the imaginarypart (k) is less than 0.15 wherein the metallized formazan dye has a kvalue of 0.00 to 0.02. For convenience the metallized formazan dyes usedin this invention are sometimes referred herein as "no k" formazans.

The optical recording elements have improved incubation stability andlight stability compared to prior art elements containing similarcomponents. It was observed that the no k formazans of the inventionprovide increased stabilization of the composition against light fade.This results because increased concentrations of no k formazans can beused compared to formazans having a higher k. Nevertheless, reflectivityis maintained or even increased. The use of no k metallized formazansdefined above allows the use of greater concentrations of metallizedformazans without adversely affecting the targeted k value for therecording layer.

DETAILED DESCRIPTION OF THE INVENTION

Any cyanine dye should be useful as long as the resulting mixture hasthe refractive index attributes mentioned above. A particularly usefulgroup of such dyes are indodicarbocyanine dyes. Typicalindodicarbocyanine dyes include: ##STR1##

The no k formazans used herein are prepared according to the methodsdescribed in U.S. Pat. No. 5,294,471. Exemplary no k metallizedformazans fall within the scope of structure (I): ##STR2## wherein R₁represents alkyl of 1-12 carbons, or alkyl substituted with one or moregroups selected from the group consisting of hydroxy, acyloxy, alkoxy,aryloxy, alkylthio, arylthio, alkylsulfonyl, arylsulfonyl, thiocyano,cyano, halogen, alkoxycarbonyl, aryloxycarbonyl, acetyl, aroyl,alkylaminocarbonyl, arylaminocarbonyl, alkylaminocarbonyloxy,arylaminocarbonyloxy, acylamino, amino, alkylamino, arylamino, carboxy,trihalomethyl, alkyl, heteroaryl, alkylureido, arylureido, succinimidoand phthalimido; or aryl, or alkyl substituted aryl or alkoxysubstituted aryl; and

R₂ represents hydrogen, alkyl or alkoxy.

In the definitions of structure I, alkyl and alkoxy have 1-12 carbons(including cycloalkyl of 5-7 carbons), aryl, aryloxy and aroyl have 6-10carbon atoms in an aromatic ring, and heteroaryl has 5-10 carbon atomsin an aromatic ring having at least one heteroatom. These definitionsapply when the foregoing terms appear as suffixes or prefixes or alone.

In addition, each of the alkyl, cycloalkyl, aryl, heteroaryl and alkoxygroups may be further substituted with one or more groups chosen fromhydroxy, acyloxy, alkoxy, aryloxy, alkylthio, arylthio, alkylsulfonyl,arylsulfonyl, thiocyano, cyano, halogen, alkoxycarbonyl,aryloxycarbonyl, acetyl, aroyl, alkylaminocarbonyl, arylaminocarbonyl,alkylaminocarbonyloxy, arylaminocarbonyloxy, acylamino, amino,alkylamino, arylamino, carboxy, trihalomethyl, alkyl, aryl, heteroaryl,alkylureido, arylureido, succinimido, phthalimido and the like.

Representative dyes falling within structure I are presented in Table 1.##STR3##

                  TABLE 1    ______________________________________    Formazan    Number  R.sub.1        R.sub.2 para subst.    ______________________________________    NKF1            1              s-Butyl    NKF2            2 #STR4##      H    NKF3 k = 0.01            3 #STR5##      Methyl    NKF4            4 #STR6##      i-Propyl    NKF5 k = 0.01            3 #STR7##      Ethyl    NKF6    --CHMe.sub.2   H    NKF7            5 #STR8##                           6 #STR9##    NKF8            5 #STR10##                           7 #STR11##    NKF9            2 #STR12##                           7 #STR13##    NKF10            4 #STR14##     Ethyl    NKF11            3 #STR15##     i-Propyl    ______________________________________

Optical Recording Elements

The optical elements of the invention comprise a light transmitting,typically pregrooved substrate, a dye recording layer overlaying thesubstrate, a light reflective layer overlaying the light absorptivelayer and a protective layer overlaying the light reflective layer. Therecording process will produce marks of lower reflectivity than theunmarked areas of the disk when written and read with a diode laseremitting between 770 and 800 nm. The substituents on the dye moleculesare selected such that the real part of the complex refractive index (n)of the unwritten recording layer measured with a 780 nm light source isgreater than 1.8 and the imaginary part (k) is less than 0.15.

The substrate may be any transparent material that satisfies themechanical and optical requirements. The substrates are generallypregrooved with groove depths from 20 nm to 250 nm, groove widths 0.2 to1 μm and a pitch 0.5 to 2 μm. The preferred material is polycarbonate,other materials are glass, polymethylmethacrylate and other suitablepolymeric materials.

The preparation of the optical recording element of the invention isachieved by spin coating of the dye mixture, with or without addenda,from a suitable solvent onto a transparent substrate. For coating, thedye mixture, with or without addenda, is dissolved in a suitable solventsuch that the dye is 20 or less parts by weight to 100 parts of solventby volume. The dye recording layer of the element is then overcoatedwith a metal reflective layer under reduced pressure by resistiveheating or a sputter method and finally overcoated with a protectiveresin.

Coating solvents for the dye recording layer are selected to minimizetheir effect on the substrate. Useful solvents include alcohols,hydrocarbon halides, cellosolves, ketones. Examples of solvents are2,2,3,3-tetrafluoropropanol, tetrachloroethane, dichloromethane, methylcellosolve, ethyl cellosolve, 1-methoxy-2-propanol,4-hydroxy-4-methyl-2-pentanone. Preferred solvents are alcohols sincethey have the least effect on the preferred polycarbonate substrates.Mixtures containing these solvents can also be used.

Useful addenda for the recording layer include stabilizers, surfactants,binders and diluents.

The reflective layer can be any of the metals conventionally used foroptical recording materials. Useful metals can be vacuum evaporated orsputtered and include gold, silver, aluminum, copper and alloys thereof.

The protective layer over the reflective layer is similarly conventionalfor this art. Useful materials include UV curable acrylates.

One preferred protective layer is disclosed in U.S. Pat. No. 5,312,663.This patent discloses a two layer structure in which the layer adjacentto the reflective layer is spin coated and the second layer is screenprinted.

An intermediate layer, to protect the metal layer from oxidation, canalso be present.

The element of the invention can have prerecorded ROM areas as describedin U.S. Pat. No. 4,940,618. The surface of the substrate can have aseparate heat deformable layer as described in U.S. Pat. No. 4,990,388.Other patents relating to recordable CD type elements are U.S. Pat. Nos.5,009,818; 5,080,946; 5,090,009; 4,577,291; 5,075,147; and 5,079,135.

Examples of representative metallized formazans from Table 1 werecombined with a cyanine dye to form optical recording layers and opticalrecording elements. For each of the recording layer mixtures studied,the cyanine dye to formazan ratio was varied so as to form an elementcapable of achieving the required refractive indices, the aim being tomeet the 70% minimum reflectivity for optical data discs published inInternational Standard ISO/IEC 10149. The combined formazan cyanine dyemixture was dissolved in an appropriate solvent (typically2,2,3,3-tetrafluoro-1-propanol (TFP)) and spun coated onto siliconwafers. Spectroscopic ellipsometric analysis of the coated wafersprovided the refractive index. The film thickness corresponding to thefirst reflective maximum and the reflectivity at that thickness werecalculated. The ratio of components was varied until the requiredreflectivity at 780 nm was achieved.

Measurement of a single component was made by coating the singlecomponent, such as a dye, as described above. K was determined byspectroscopic ellipsometric analysis also as described above. In somecases, a dye would crystallize preventing measurement. In the case ofthe metal formazans, we were able to measure k for only two dyes. Sincethe other dyes in this class are similar in structure to the dyes forwhich k measurements were obtained, and such dyes performed similarly inoptical recording elements to the dyes for which k measurements wereobtained, it is assumed that they have no k values (k less than 0.02) aswell.

The following examples demonstrate the light and incubation stability ofthe optical recording layer used in the elements of this invention.

For each of the dye mixtures studied, an optical recording element wasprepared by first dissolving the dye mixture in an appropriate solvent.The solvent mixture was MP/TCE 1-methoxy-2-propanol and2,2,2-trichloroethanol (97:3)! unless otherwise stipulated. The solutionwas spin coated onto a 200 nm grooved polycarbonate substrate usingmethods well known to those skilled in the spin coating art. A completerecording element was prepared by applying 70 to 100 nm of gold to thedye film by sputtering and the gold layer was overcoated with 5 micronsof a photo-crosslinkable lacquer (Daicure SD-17). The element was testedby recording an EFM pattern using a commercially available media tester(MT-16 from Philips NV) and the pattern read back on a CD-CATS reader(commercially available from Audio Development Informationsteknik ABSweden).

The light stability of the coated dye mixtures in optical recordingelements was evaluated in some examples by preparing appropriatemixtures of no k formazans with a cyanine dye. The mixture was dissolvedin 2,2,3,3-tetrafluoro-1-propanol at 2% solids and spun coated on 2"×2"pieces of polycarbonate at 600 rpm. After drying, the spectra of theresulting recording layers was measured and the absorbance at thewavelength corresponding to that of maximum absorbance of CD-1 (714 nm)was recorded. The recording layers were then exposed to a 50 Klux highintensity daylight source for periods extending to 14 days, theabsorbance loss (at 714 nm) noted after each increment of exposure. Incontrol compositions containing only CD-1 (no formazan dyes) the losswas 100 percent after three days exposure.

The criteria for incubational stability, 2 weeks at 80° C./85% RH andlight stability, 5 days of 50 Klux light exposure are that contrast(I11R) not drop by more than 0.05 after 2 weeks incubation, thatreflectivity not drop below 65%, that symmetry not increase or decreaseby more than 10 units, and that BLER not rise above 50. BLER is errorrate.

COMPARATIVE EXAMPLE

A dye mixture containing 4 parts of CD-1 and 1 part of a prior artnickel formazan was dissolved in 1-methoxy-2-propanol. The nickelformazan has a strong electron withdrawing group. It has a structureaccording to structure I in which R₁ =3-C₇ H₁₅ and R₂ =4-NO₂. Themixture was coated on a 45 nm deep grooved substrate. An opticalrecording element was formed therefrom and tested as described aboveboth fresh and after 1 week incubation at 80C/85% RH. The element ofthis comparative example was incubationally unstable

Examples 1-3

Three different recording elements having recording layers formed withdye mixtures containing cyanine dye CD-1 and no k formazans NKF2, NKF3and NKF4 at ratios of both 4 to 1 and 6 to 4 were prepared as in thecomparative example. Improved light stability while maintaining therequired reflectivity was noted for the 6:4 mixtures as compared to the4:1 ratio used in the comparative example. All further examples were runat the 6:4 ratio.

Examples 4-6

Dye mixtures containing 6 parts of CD-1 and 4 parts of three differentno k metallized formazans from Table 1 were dissolved in1-methoxy-2-propanol, coated on a 50 nm deep grooved substrate andincorporated into an optical recording element. The no k metallizedformazans were NKF2, NKF4 and NKF5. The elements were tested forincubational stability for two weeks and found to be stable. These sameelements were found to be stable after 5 days of light exposure.

Examples 7-9

Examples 4-6 were repeated using no k formazans NKF1, NKF3 and NKF6.Light and incubational stability similar to that observed in examples4-6 were noted.

This invention has been described with particular reference to preferredembodiments thereof but it will be understood that modifications can bemade within the spirit and scope of the invention.

What is claimed is:
 1. An optical recording element having a transparentsubstrate and on the surface of said substrate, a recording layer and alight reflecting layer; whereinthe recording layer comprises a mixtureof a metallized formazan dye and cyanine dye and the unrecorded layer issuch that the real part of the refractive (n) at 780 nm is greater than1.8 and the imaginary part (k) is less than 0.5; and the metallizedformazan dye has (a) a k value of 0.00 to 0.02 and (b) a structure I asfollows: ##STR16## wherein R₁ represents alkyl of 1-12 carbons, or alkylsubstituted with one or more groups selected from the group consistingof hydroxy, acyloxy, alkoxy, aryloxy, alkylthio, arylthio,alkylsulfonyl, arylsulfonyl, thiocyano, cyano, halogen, alkoxycarbonyl,aryloxycarbonyl, acetyl, aroyl, alkylaminyocarbonyl,arylamininocarbonyl, alkylaminocarbonyloxy, arylaminocarbonyloxy,acylamino, amino, alkylamino, arylamino, carboxy, trihalomethyl, alkyl,heteroaryl, alkylureido, arylureido, succinimido and phthalimido; oraryl, or alkyl substituted aryl or alkoxy substituted aryl; and R₂represents hydrogen, alkyl or alkoxy.
 2. The element of claim 1whereinR₁ represents phenyl, methyl phenyl, ethoxyphenyl and i-propyl;and R₂ represents hydrogen, methyl, ethyl, i-propyl, n-butyl, s-butyl ormorpholinomethyl.
 3. The element of claim 2 wherein the metallizedformazan is selected from Table 1: ##STR17##

                  TABLE 1    ______________________________________    Formazan    Number  R.sub.1        R.sub.2 para subst.    ______________________________________    NKF1            1              s-Butyl    NKF2            2 #STR18##     H    NKF3 k = 0.01            3 #STR19##     Methyl    NKF4            4 #STR20##     i-Propyl    NKF5 k = 0.01            3 #STR21##     Ethyl    NKF6    --CHMe.sub.2   H    NKF7            5 #STR22##                           6 #STR23##    NKF8            5 #STR24##                           7 #STR25##    NKF9            2 #STR26##                           7 #STR27##    NKF10            4 #STR28##     Ethyl    NKF11            3 #STR29##     i-Propyl    ______________________________________


4. The formazan of claim 2 selected from formazans numbers NKF 1, NKF2,NKF3, NKF4, NKF5, NKF6 and NKF11 of Table
 1. 5. An element according toany one of the preceding claims 1-4 wherein the cyanide dye is anindodicarbocyanine dye.
 6. An element according to any one of claims 1-4wherein the cyanine dyes are selected from the group consisting of##STR30##
 7. The element of any one of claims 1-4 wherein the cyaninedye has the structure
 8. The element of claim 3 wherein the cyanine dyeto metallized formazan dye ratio is 6:4.